feat: automatic forced logout on expired session

When both access and refresh token are invalid (401 on /auth/refresh),
the app now automatically logs out and navigates to Settings (login form).
No data loss - only auth tokens are cleared, local inventory data is intact.

- AuthEventBus: singleton SharedFlow that signals session expiry
- MainViewModel: observes bus, calls logout + disconnect, navigates to Settings
- MainScreen: LaunchedEffect collects navigateToSettings event
- MessageRepositoryImpl: emits session expired when refresh fails
- SyncServiceImpl: emits session expired when refresh fails

app/src/main/java/de/bollwerk/app/data/repository/MessageRepositoryImpl.kt

@@ -9,6 +9,7 @@ import de.bollwerk.app.data.sync.RefreshRequest
 import de.bollwerk.app.data.sync.WebSocketClient
 import de.bollwerk.app.data.sync.WebSocketEvent
 import de.bollwerk.app.di.ApplicationScope
+import de.bollwerk.app.domain.AuthEventBus
 import de.bollwerk.app.domain.model.SettingsKey.StringKey
 import de.bollwerk.app.domain.model.SyncError
 import de.bollwerk.app.domain.repository.MessageRepository
@@ -54,6 +55,7 @@ internal class MessageRepositoryImpl @Inject constructor(
     private val settingsRepository: SettingsRepository,
     private val webSocketClient: WebSocketClient,
     private val e2eeKeyManager: E2EEKeyManager,
+    private val authEventBus: AuthEventBus,
     @ApplicationScope private val scope: CoroutineScope
 ) : MessageRepository {
 
@@ -222,6 +224,7 @@ internal class MessageRepositoryImpl @Inject constructor(
                 settingsRepository.setString(StringKey.AuthRefreshToken, loginResponse.refreshToken)
                 true
             } else {
+                authEventBus.notifySessionExpired()
                 false
             }
         } catch (_: Exception) {

app/src/main/java/de/bollwerk/app/data/sync/SyncServiceImpl.kt

@@ -1,5 +1,6 @@
 package de.bollwerk.app.data.sync
 
+import de.bollwerk.app.domain.AuthEventBus
 import de.bollwerk.app.domain.model.SettingsKey.StringKey
 import de.bollwerk.app.domain.model.SyncError
 import de.bollwerk.app.domain.repository.SettingsRepository
@@ -29,7 +30,8 @@ import javax.inject.Inject
 
 internal class SyncServiceImpl @Inject constructor(
     private val httpClient: HttpClient,
-    private val settingsRepository: SettingsRepository
+    private val settingsRepository: SettingsRepository,
+    private val authEventBus: AuthEventBus
 ) : SyncService {
 
     override suspend fun downloadInventory(since: Long?): Result<InventoryDto> =
@@ -139,6 +141,7 @@ internal class SyncServiceImpl @Inject constructor(
                     if (newToken.isBlank()) return@withContext result
                     block(serverUrl.trimEnd('/'), newToken)
                 } else {
+                    authEventBus.notifySessionExpired()
                     result
                 }
             } else {
@@ -189,6 +192,7 @@ internal class SyncServiceImpl @Inject constructor(
                 settingsRepository.setString(StringKey.AuthRefreshToken, loginResponse.refreshToken)
                 true
             } else {
+                authEventBus.notifySessionExpired()
                 false
             }
         } catch (_: Exception) {

app/src/main/java/de/bollwerk/app/domain/AuthEventBus.kt

@@ -0,0 +1,22 @@
+package de.bollwerk.app.domain
+
+import kotlinx.coroutines.flow.MutableSharedFlow
+import kotlinx.coroutines.flow.SharedFlow
+import kotlinx.coroutines.flow.asSharedFlow
+import javax.inject.Inject
+import javax.inject.Singleton
+
+/**
+ * Singleton-Bus für auth-kritische Ereignisse.
+ * Emittiert wenn sowohl Access- als auch Refresh-Token ungültig sind,
+ * damit die App den User automatisch ausloggt ohne Datenverlust.
+ */
+@Singleton
+internal class AuthEventBus @Inject constructor() {
+    private val _sessionExpired = MutableSharedFlow<Unit>(extraBufferCapacity = 1)
+    val sessionExpired: SharedFlow<Unit> = _sessionExpired.asSharedFlow()
+
+    fun notifySessionExpired() {
+        _sessionExpired.tryEmit(Unit)
+    }
+}

app/src/main/java/de/bollwerk/app/ui/MainScreen.kt

@@ -21,6 +21,7 @@ import androidx.compose.material3.Text
 import androidx.compose.material3.TextButton
 import androidx.compose.material3.TopAppBar
 import androidx.compose.runtime.Composable
+import androidx.compose.runtime.LaunchedEffect
 import androidx.compose.runtime.getValue
 import androidx.compose.runtime.mutableStateOf
 import androidx.compose.runtime.remember
@@ -37,6 +38,7 @@ import androidx.navigation.compose.rememberNavController
 import de.bollwerk.app.ui.inventory.InventoryPickerSheet
 import de.bollwerk.app.ui.inventory.InventoryPickerViewModel
 import de.bollwerk.app.ui.navigation.BollwerkNavGraph
+import de.bollwerk.app.ui.navigation.Screen
 import de.bollwerk.app.ui.navigation.TopLevelDestination
 import de.bollwerk.app.ui.update.UpdateBanner
 import de.bollwerk.app.ui.update.UpdateStatus
@@ -53,6 +55,16 @@ internal fun MainScreen() {
         currentDestination?.hasRoute(topLevel.route::class) == true
     }
 
+    val mainViewModel: MainViewModel = hiltViewModel()
+    LaunchedEffect(Unit) {
+        mainViewModel.navigateToSettings.collect {
+            navController.navigate(Screen.Settings) {
+                popUpTo(navController.graph.startDestinationId) { inclusive = false }
+                launchSingleTop = true
+            }
+        }
+    }
+
     val inventoryPickerViewModel: InventoryPickerViewModel = hiltViewModel()
     val inventoryState by inventoryPickerViewModel.uiState.collectAsStateWithLifecycle()
     var isInventoryPickerVisible by remember { mutableStateOf(false) }

app/src/main/java/de/bollwerk/app/ui/MainViewModel.kt

@@ -0,0 +1,36 @@
+package de.bollwerk.app.ui
+
+import androidx.lifecycle.ViewModel
+import androidx.lifecycle.viewModelScope
+import dagger.hilt.android.lifecycle.HiltViewModel
+import de.bollwerk.app.data.sync.WebSocketClient
+import de.bollwerk.app.domain.AuthEventBus
+import de.bollwerk.app.domain.repository.SyncService
+import kotlinx.coroutines.flow.MutableSharedFlow
+import kotlinx.coroutines.flow.SharedFlow
+import kotlinx.coroutines.flow.asSharedFlow
+import kotlinx.coroutines.launch
+import javax.inject.Inject
+
+@HiltViewModel
+internal class MainViewModel @Inject constructor(
+    private val authEventBus: AuthEventBus,
+    private val syncService: SyncService,
+    private val webSocketClient: WebSocketClient
+) : ViewModel() {
+
+    private val _navigateToSettings = MutableSharedFlow<Unit>(extraBufferCapacity = 1)
+    val navigateToSettings: SharedFlow<Unit> = _navigateToSettings.asSharedFlow()
+
+    init {
+        viewModelScope.launch {
+            authEventBus.sessionExpired.collect {
+                // Tokens löschen (kein Datenverlust – nur Auth-Daten)
+                syncService.logout()
+                webSocketClient.disconnect()
+                // Navigation zu Settings (Login-Formular)
+                _navigateToSettings.emit(Unit)
+            }
+        }
+    }
+}